From your profile you seem to be run-in OS X 10.11 which is El Capitan, there is no Manage button
in the Storage section, that option only came to macs with later OS's.
If you want to free up space on your mac just offload files you do not need on a daily basis to an external drive.
Movies, Images and Music commonly taken up the most space. Once you have confirmed thatchy have copied over
successfully you can delete them from your mac.
Before upgrading to Catalina I would recommend upgrading to macOS High Sierra first.
Which OS you can upgrade to will be governed by which model and year your mac is.
Click on the top left of your screen click on About This Mac to find out the year of your mac.
For High Sierra and Catalina,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to Check Compatibility to see if your mac qualifies for the OS upgrade you want.
Then go to the Download OS section and click on the OS you want to install.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you want to or are able to install Big Sur, open Safari and click on the link below.
How to upgrade to macOS Big Sur - Apple Support
If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are offering 64bit updates.